Polymer electrolytes have attracted great interest because of their potential in the development of new technologies such as polymer batteries, fuel cells and sensors. For example, the lithium ion battery provides high energy density due to the small atomic weight and the large ionization energy of lithium and has become widely used as a power source for many portable electronics such as cellular phones, laptop computers, mini-cameras, etc. In a battery cell, an electron insulating separator is used to separate the two electrodes and electrolytes are used to facilitate the necessary passage of ions between reduction and oxidation sites. Membranes made from polymeric electrolytes can serve as a combination of separator and electrolyte providing greater efficiency in manufacture of the batteries while addressing growing concerns about their safety by eliminating the flammable organic solvents used in many electrolyte systems.
Armand et al. found in 1978 that poly(ethylene oxide) (PEO) can dissolve lithium perchlorate salts forming a complex that can serve as a solid electrolyte. This complex has a relatively good ionic conductivity in a solid state. However, the ionic conductivity is insufficient as compared with the ionic conductivity of the non aqueous electrolyte solution and the cation transport number of the complex is extremely low.
Subsequently a broad range polymer electrolytes composed of linear polyethylene oxide related polymers such as polypropylene oxide, polyethyleneimine, polyurethane and polyester were developed. In general, these polymer electrolytes have an ionic conductivity at room temperature of about 10−7 to 10−6 S/cm. It is believed that ion conduction is due mainly to the amorphous portion of a polyether polymer which conducts ions by local movement of polymer chain segments, Armand, Solid State Ionics, Vol. 9/10, p. 745-754, 1983. A linear polyether polymer such as PEO tends to crystallize with metallic salt dissolved therein and thus limits ion movement so that actual ion conductivity is much lower than the predicted value.
For a polymer electrolyte to offer high ion conductivity, it ideally should have many amorphous regions of good ion conductor mobility and not crystallize even in the presence of a high concentration of dissolved ion-conductive salt. Many attempts to design and make branched PEOs for electrolytes have been reported, for example, Atsushi Nishimoto et al Electrochim Acta, Vol. 43, No. 10-11, pp. 1177-1184, 1998, however, the synthesis is complicated and manufacturing costs are high.
Furthermore, the cation transport number of the PEO complex tends to be extremely low due to a strong interaction of cations with polar groups of the matrix polymer relative to that of anions. In a secondary battery, the electrodes used are active to cations. When an electrolyte with both moveable cations and anions is used, the movement of anions can be interrupted by the cathode resulting in a concentration polarization which may cause fluctuations in voltage or output of the secondary battery. Thus, an ionic conductor having non-moveable anions is desirable. Such material is often referred to as single-ion conductor and generally has a high cation transport number.
U.S. Published Pat. Appl. 2006/0014066A1, incorporated herein in its entirety by reference, discloses an ion conductive polymer having stereospecific structures which are claimed to improve ion conductivity and cationic transport number. The syndiotactic poly(styrene sulfonate) solid polymer electrolyte disclosed therein has a room temperature (25 C) conductivity of 2×10−6 S/cm, which is higher than conventional solid conductive polymer electrolytes.
U.S. Pat. No. 6,537,468, incorporated herein in its entirety by reference, discloses a polymer electrolyte with high conductivity comprising a polymer containing polyvinyl alcohol units substituted or grafted with oxyakylene and a crosslinker to form a semi-interpenetrating network (IPN) structure wherein a branched oxyalkylene polymer confined in an IPN contributes high ion conductivity and the desired tackiness while the crosslinked structure provides shape retention. The conducting branched polyvinyl alcohol is not covalently linked to the crosslinked network. Solid polymer electrolytes with tackiness were formed by heating (e.g., at 100° C.) to achieve crosslinking.
U.S. Pat. No. 6,881,820, incorporated herein in its entirety by reference, discloses a series of rod-coil block polyimide copolymers formed by condensation polymerization of a tetracarboxylic acid dianhydride with a polyoxyalkylene diamine and an aromatic polyfunctional amine that can be fabricated into mechanically resilient film with acceptable ionic or protonic conductivity at variety temperatures. The copolymers consist of short-rigid polyimide rod segments alternating with polyether coil segments. Preparation of copolymers with graft or comb structures consisting of a rigid polymer backbone with polyether coil graft segments is not disclosed, nor is preparation of crosslinked copolymers using polyfunctional anhydride such as styrene and maleic anhydride copolymer with more than two anhydride functional groups.
U.S. Pat. No. 6,368,746, incorporated herein in its entirety by reference, discloses a molded solid electrolyte consisting of an inorganic solid electrolyte such as lithium sulfide (Li2S) and silicon sulfide (SiS2) with an organic binder polymer such as polybutadiene copolymer rubber.
In spite of these efforts, current polymer electrolytes consisting of branched or grafted oxyalkylene polymers to increase the amorphous portion of the polymer and improve conductivity still fail to provide the ion conductivity desired and typically involve complicated syntheses.
Attempts to develop alternate single ion conductive polymers, for example, with fixed anion groups to raise cation transference number and reduce polarization, have encountered similar drawbacks. High-porous membranes capable of encapsulating a liquid electrolyte in its porous structure were exploited for improved mechanical stability. However, there may be disadvantages of poor contact with electrodes and possibility of liquid leakage. Lithium ion conductive inorganic solid electrolytes were also exploited but require high temperatures to make and are not flexible unless used together with organic polymers.
To achieve high conductivity, gel polymer electrolytes (GPE) using soluble polymers such as poly(ethylene oxide) (PEO), PVC, poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVDF)PEO, were developed for use in polymer Li ion batteries. GPE's potentially have ionic conductivity near the order of magnitude of liquid electrolytes (>10−4 S/cm possible).
Many of these gel polymer electrolytes typically have either high conductivity with poor mechanical properties or good mechanical properties with low conductivity. PEO-based electrolytes have a high degree of crystallinity unfavorably affecting its ionic conduction. PAN-based electrolytes undergo solvent exudation upon long storage and solvent will ooze to the surface of the polymer membrane decreasing the ionic conductivity dramatically. PMMA-based electrolytes have a high ionic conductivity but it is difficult to form a polymer membrane with stable dimensions and good physical properties.
PVDF with its high dielectric constant (∈=0.84) and strong electro-withdrawing functional groups is non-soluble but swellable in carbonate solvents and therefore can provide a supporting structural phase with good mechanical strength when used as the polymer matrix for GPE.
There are many potential advantages in the use of solid polymer electrolytes, for example, adjustable physical properties such as flexibility, rigidity, processability, softness, hardness, porosity, tackiness etc, low toxicity, minimal fire hazard, light weight, high energy density, lower manufacturing costs, improved performance, etc. However, there remains a need for new solid electrolytes having unique molecular architecture and/or new ion transport mechanisms that can provide good ion conductivity at room temperature with no solvent. For example, room temperature conductivity of conventional solid conductive polymer electrolytes is 1×10−6 s/cm or lower which is insufficient for many applications.